1. Field of the Invention
The present invention relates to a mark for a visual inspection upon assembling a display, and also to an electronic equipment provided with a display panel, a tape automated bonding (TAB) substrate, or a flexible substrate having the mark for the visual inspection thereon, respectively.
The present application claims priority from Japanese Patent Application No. 2001-279532, the disclosure of which is incorporated herein by reference for all purposes.
2. Description of the Related Prior Art
Recently, a portable telephone or a personal digital assistant (PDA) has come into wide use, and needs a display panel which has a small size, reduced thickness, and high display capability. A liquid crystal display panel or an organic electro-luminescent (EL) display panel is employed as the display panel meeting with the requirement.
In order to improve the display capability of such a display panel, an optical control characteristic or a light emission characteristic of each display element, of course, must be improved, and additionally it is also an important factor to bond electronic parts onto the display panel with high accuracy.
With reference to FIG. 5, the conventional bonding process of the electronic parts onto the display panel will be described. The electronic parts such as the TAB substrate and a flexible printed circuit (FPC) are connected onto the display panel. Here, the connection between the display panel and the TAB substrate will be explained as an example.
A panel substrate 51 of a display panel 50 has leads 51a of the display panel 50 to be exposed at an end portion of the substrate 51. The leads 51a are connected to leads 52a of the TAB substrate 52 through an anisotropic conductive film (ACF) 53. Alignment marks 51b are provided on lateral both sides of a connecting region formed by the leads 51a of the panel substrate 51. Likewise, alignment marks 52b are provided on the TAB substrate 52 to fit with the alignment marks 51b. A photograph of these alignment marks 51b and 52b is taken by an image pickup device 54, and is observed by an enlargement thereof or image-processing. In such a way, positioning is regulated by a regulating mechanism (not shown) so as to bring the positions of the both display panel 50 and the TAB substrate 52 into a predetermined position-relationship therebetween.
After completing the positioning, the end portion of the panel substrate 51 is overlapped with an end portion of the TAB substrate 52 through the ACF 53. The overlapped portion is pressed down by a temporary pressure-bonding head 55. Thus, the both panel substrate 51 and the TAB substrate 52 are temporarily bonded, putting the ACF 53 between the both.
After, then, completing the temporary bonding, the positioning of the alignment marks and also dust or the like to be adhered on the panel are inspected. Only the display panel which passed in these inspections is conveyed into a permanent pressure-bonding process. In the permanent pressure-bonding process, a predetermined temperature and pressure is applied to the temporary bonding portion during a predetermined time, thereby permanently bonding the panel substrate 51 with the TAB substrate 52.
Even if the positioning is performed by using the above-mentioned alignment marks and image processing of the alignment marks through the image pickup device, it is very difficult to perform perfect positioning of each lead disposed with high density since there may be any problem of resolution ability in a photographic image, or any formation error of a substrate glass, the TAB substrate, the FPC, and the like, respectively. In addition, a positional deviation or a pressure-bonding failure may occur in the temporary bonding step or the permanent bonding step after the positioning is performed, so that a connection failure may be caused.
In particular, in the case of a display panel comprising current injection type display elements such as an organic EL display panel, a minute positional deviation causes a contact area formed by connected leads to be decreased, so that there may occur a bad effect to the injected current value. Specifically, even if the connection between each lead is secured, the display performance may be deteriorated due to reduction of the contact area. Comparing with a liquid crystal display panel or the like to be driven by a voltage, the organic EL display panel requires strict accuracy of the positioning. Then, in the case of the organic EL display panel, a visual inspection is performed after the panel substrate is permanently bonded with pressure to the TAB substrate in order to firstly eliminate inferior products which can be visually found out. Further, a microscopic inspection is performed to the remaining products in order to inspect whether the contact area between leads is appropriately secured.
Here, since the microscopic inspection is a work with high load, it is preferable that the inferior products are eliminated in the visual inspection prior to the microscopic inspection as much as possible, and as a result, the objects of the microscopic inspection should be decreased. However, it is impossible for the visual inspection to perfectly examine to see whether the contact area between the leads is under a good or bad condition. In addition, the above-mentioned alignment marks also for regulating the position of the both substrates are too small to find the inferior products by the visual inspection. Thus, in a conventional way, only inferior products having obvious problems such as a distinct pressure-bonding failure can be eliminated by the visual inspection step. As a result, the workload in the microscopic inspection step can not be reduced.